Process
Generally, the permanent magnets may be disposed at the surface, directly facing the air gap, or, in a variant, may be disposed within the magnetic mass, in housings therein, the rotor then being classed as a “rotor with buried magnets”.
In this latter case, it is necessary to assure a radial and/or axial mechanical blocking of the magnets in their housings, wherein this blocking must be sufficient in order to prevent damage to the magnets and allow correct operation of the machine. In effect, in the event of insufficient chocking, the magnets may be subjected to micro-displacements, which may lead to the destruction of the magnets, to a degradation of the electric and magnetic performances of the machine, and to a balancing fault.
In order to fix the magnets in their housings, a number of techniques are currently used, such as the use of glue, the use of a specific magnet shape and a specific shape of the corresponding housing, for example the use of magnets having a trapezoidal cross section, the impregnation of the magnet in its housing following the placement of said magnet, or blocking by addition of a deformable piece, such as a chock. However, these techniques have some disadvantages. Their implementation may be awkward and costly. For example, a process in which the magnets are glued in the housings may be inconvenient with regard to the working conditions, may pose a problem of durability over the assembly time for some applications, and makes the recovery of the magnets practically impossible without damage.
With regard to impregnation, this is a long and very costly process, which is also cumbersome in terms of implementation, taking into account the need to use containers of varnish and ovens. In addition, this imposes a thermal constraint linked to the demagnetization of the magnets (the temperature of impregnation having to be lower than the thermal limit resulting in a demagnetization of the magnets) and also makes the recovery of the magnets impossible without damage.
The use of magnets having a specific shape is not always possible insofar as such shapes may complicate the provision of the magnetic sheet and may increase the cost of the cutting tools and therefore the total production cost of the machine.
Influence on Performance
An optimal electromagnetic performance is obtained when a buried magnet is in perfect contact at each of its two north and south polar faces with the magnetic mass in which it is inserted, the passage of the magnetic flux from the magnets to the magnetic mass being maximized.
However, there is in general a play between the magnets and their housings in the magnetic mass in which said magnets are inserted, thus constituting an air gap from a magnetic viewpoint, which necessarily results in a reduction of the electromagnetic performance of the machine. A play of this type is linked to the manufacturing constraints, which do not make it possible, for a reasonable cost, to observe very precise dimensions when cutting the magnetic mass or when designing the magnets. A play may also be caused by the fact that, because the magnets are sensitive to corrosion, it may be necessary to cover the magnets with a protective coating, which also results in an uncertainty with regard to the dimensions of said magnets.
In addition, the assembly constraints make it necessary to retain a certain play between the magnets and the housings of the magnetic mass, so as to facilitate the insertion of the magnets into the latter, in particular when the magnetic mass is formed by a stack of thin magnetic sheets. In effect, in this case, the walls of the magnetic mass may not be perfectly straight, taking into account the fact that they are constituted by a stack of thin sheets, which may necessitate an even greater assembly play. Conventionally, a tolerance range of +/−1-0.2 mm may be provided in the dimensions of the magnets. In addition, it may be necessary to provide an assembly play reaching 0.1 mm, even better 0.15 mm on either side of the magnets for the assembly. Ultimately, a play of approximately 0.25 to 0.5 mm, preferably from 0.25 to 0.35 mm per magnet may be obtained conventionally, which is far from negligible relative to the usual air gaps between rotor and stator currently observed in low to medium power electric machines, which are approximately from 0.5 mm to 1 mm.
In the case in which the machine comprises a plurality of magnets disposed in a number of rows per pole in the magnetic mass, the plays of the magnets in different rows are added together and further reduce the magnetic performance of the machine.
Application JP 2007-336671 discloses magnets having cavities which engage with lugs present in housings.
U.S. Pat. No. 5,679,995 discloses housings comprising lugs on their edges. These lugs bend or deform plastically in order to hold the permanent magnets in the housings.
Application JP 2004-328819 discloses the use of springs in housings in order to clamp magnets therein. The application US 2013/0334910 describes housings having grooves in order to facilitate the passage of the fixing resin along the edges of the magnets.
There is a need to further improve the magnetic performance of rotary machines with permanent magnets and to reduce the manufacturing and assembly costs thereof.